The present invention relates to a control apparatus for an elevator in which a cage is driven by an A.C. electric motor, and more particularly to the prevention of vibrations of the cage.
FIG. 1 is a block diagram showing the arrangement of a control apparatus for a conventional elevator in which a cage is driven by an induction motor, along with a mechanical system including the cage. A three-phase power source 1 has a converter for power running 2 and a converter for regenerative braking 3 connected in parallel therewith, a smoothing capacitor 4 and an inverter 5 are connected in parallel across the output terminals of these converters, and an induction motor 6 is connected to the inverter 5. Although, in FIG. 1, only a circuit corresponding to one phase of a three-phase alternating current is illustrated for any of the converters 2 and 3 and the inverters 5 similar such circuits are actually connected in parallel for the three phases. Coupled to the induction motor 6 are a tachometer generator 7 which serves as a speed detector and a sheave 8. A brake 9 is disposed on the outer side of the sheave 8, round which a hoisting rope 12 is extended. A cage 10 is coupled to one end of this hoisting rope, and a balance weight 11 to the other end.
On the other hand, in order to detect the position of the cage 10, a governor rope 16 is wound round a governor 14 comprising a pulse generator 15. The governor rope 16 has its ends joined to the upper end part and lower end part of the cage 10, respectively, and is subjected to a tension by a tightening pulley 17.
In addition, in order to control the aforementioned converters 2, 3 and inverter 5, a voltage/frequency control unit (hereinbelow, simply termed "control unit") 13 is disposed, which is supplied with the speed signal of the cage 10 detected by the tachometer generator 7 and the pulse signal of the pulse generator 15 for detecting the position of the cage. The control unit 13 is constructed of a microprocessor (CPU), an interface (I/F), a random access memory (RAM), a read only memory (ROM), etc.
In the arrangement of FIG. 1, during the power running mode of operation, a three-phase A.C. voltage is converted into a D.C. voltage by the converter for power running 2, and the D.C. voltage is smoothed by the capacitor 4 and then applied to the inverter 5. Here, when the control unit 13 controls the inverter 5 in accordance with the pulse width modulation (PWM) system, an A.C. voltage of any desired frequency and voltage values is produced from the inverter 5. Such control is called the variable-voltage and variable-frequency control (VVVF control). Here, when the control unit 13 controls the inverters 5 in a phase sequence corresponding to the operating direction of the elevator, the induction motor 6 is started in a direction conforming to the phase sequence, and the cage 10 begins to run. Thenceforth, the signals of the tachometer generator 7 and the pulse generator 15 are fed to the control unit 13, so that the running control is done in the state in which position feedback is always applied.
In the elevator of this sort, as the cage rises higher, the resonance frequency of the mechanical resonance system including the cage 10, balance weight 11, hoisting rope 12, etc., becomes lower, and the speed control system becomes easier to oscillate because the vibrations of the mechanical system change the output signal of the tachometer generator 7 and interfere with the speed control system.
This is attributed to the fact that the natural frequency of the mechanical system which is constructed of the sheave 8, cage 10, balance weight 11 and hoisting rope 12 becomes close to the cutoff frequency of the motor control system. When the gain of the speed control system has increased near the natural frequency of the mechanical system, oscillations occur if the speed control system itself has no phase margin.
Such oscillations violently vibrate the cage 10 in the vertical direction, and, accordingly, does not provide a good ride. Even in a case where the oscillation is not reached, the control system becomes sensitive to a disturbance because of the increased gain of the whole control system. As a result, even a slight torque ripple of the induction motor 6 cannot be suppressed, and vibrations which disturb the riding conditions of passengers arise.
Since, in this case, the induction motor 6 is driven by the inverters 5, a torque ripple of 6.multidot.n.multidot.f (n=1, 2, 3, . . . ) where f denotes the output frequency of the inverters 5 develops in the induction motor 6, to lead to the situation in which the foregoing oscillation is liable to occur.
Here, a large number of vibrational aspects are predicted as to the mechanical system. Particularly problematic due to a slow attenuation is the vibrational aspect in which the cage 10 and the balance weight 11 are substantially at a stop, with only the seave 8 rotating violently.
FIG. 2 shows an equivalent model for analyzing the vibrations of the mechanical system. Symbol M.sub.1 indicates the mass of the cage 10, symbol M.sub.2 the equivalent combined mass of the induction motor 6 and the sheave 8, symbol M.sub.3 the mass of the balance weight 11, symbol K.sub.1 the spring constant of the cage side rope 12A, symbol K.sub.2 the spring constant of the balance weight side rope 12B, symbol C.sub.1 the attenuation constant of the cage side rope 12A, and symbol C.sub.2 the attenuation constant of the balance weight side rope 12B.
Assuming that the cage 10 and the balance weight 11 be substantially at a stop, this equivalent model can be substituted by an equivalent model which is fixed at both the ends as illustrated in FIG. 3.
In the equivalent model of FIG. 3, letting x.sub.2 denote the variation of the mass M.sub.2 from the equilibrium point thereof, the following equation of motion approximately holds: EQU M.sub.2 .multidot.x.sub.2 +(C.sub.1 +C.sub.2).multidot.x.sub.2 +(K.sub.1 +K.sub.2).multidot.x.sub.2 =0 (1)
When the attenuation constant (C.sub.1 +C.sub.2) of the second term in Equation (1) is set to be large, naturally the sharpness of resonance lowers. As a practicable method, therefore, it has been proposed to indirectly damp the rope system (refer to the official gazette of Japanese Utility Model Registration Application Publication No. 56-4701).
Since, however, this method applies indirect damping to the rotating mode of the sheave 8 through the rope, it has not always achieved a great effect.
As another method, there is proposed a damper of Lanchester which is disposed for the rotating mode of the sheave 8. This method, however, has had the disadvantages that the apparatus becomes large in size and that a mechanical loss is always involved.